Electron discharge tube



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ATTORNEY Patented Aug. 2, 1938 UNITED-STATES PATENT @FFHQE ELECTRONDISCHARGE TUBE John Henry Owen Harries, Frinton-on-Sea, EnglandApplication December 12, 1936, Serial No. 115,509 In Great BritainAugust 24, 1934 14 Claims. (01. 250- 275) This invention relates toelectron discharge tubes, and comprises matter divided from applicationfor Letters Patent, Serial No. 47,042, October 28th, 1935.

The main object of this invention is to provide a form of discharge tubecapable of use in ious stages of a radio receiver or similar apparatus,such as thermionic amplifiers for television work, and indeed in theextreme case to pr a tube which may be employed, so to speak,

ovide as a universal or all-stage tube, whereby a single embodiment maybe employed without any alteration in each of the several stages of amulti-tube radio receiver; furthermore, the invention aims at producinga form of discharge tube which is very convenient for use as the tube ina single tube frequency converter such as is employ supersonicheterodyne receivers. i

ed in a radio receiver.

First of all, in asingle tube frequency converter stage of a supersonicheterodyne receiver, the one tube has to serve as a detector as well asa generator of the local oscillations.

In particular it is found necessary to provide for very completeelectrical separation between the signal frequency and the oscillationcircuits. The oscillator be stable and its frequency shift with gainmust con-

trol must be negligible. In modern receivers it is desirable to providefor automatic gain control which can be regulated so as toprovideapproximately zero' gain without disturbing the local oscillations.Again, the anode impedance should not be less than one million ohms,while the initial anode current should be as small as possible,preferably not greater than 1.5 to 2.0 milliamperes at maximum gain.Finally, the conversion conductance should be as high as is compatiblewith low cross-modulation which should be a maximum at high gain.

The present invention aims at providing very complete separation betweenthe oscillator and signal frequency circuits without employing the usualmethod of screening in the tube itself, and thus by means largely, ifnot entirely, independent of frequency changes.

The requirements in a tube to enable it toact satisfactorily in theintermediate frequency and in the radio frequency amplifier stages arebriefly as follows:-

The anode to control grid capacity should be very small. It is foundthat with modern high gain tubes the anode to control grid capacityshould certainly not be greater than 0.02 mmfd. to 0.005 mmfd. foroperation at intermediate frequencies of the order of 110 kilocycles persecond. At higher intermediate frequencies of the order of 450kilocycles per second, instability commences toappear, and then theanode to control grid capacity should not be greater than about 0.0015mmfd. Again, the anode impedance should not be less than one millionohms. Theoretically it should not be less than five times the anodeload. If it is less than one million ohms, selectivity and amplificationare adversely affected in practice. The initial anode current should beof the order of 7.5 milliamperes. The screen current should be as low aspossible and not more than about one quarter to one third of the initialanode current. It is not desirable to have mutual conductance muchgreater than about 2 milliamperes per volt in intermediate frequencyamplifiers. Owing to commercial limitations and diificulties inscreening in radio receivers it is particularly important in radiofrequency stages that crossmodulation should be a minimum when the gainis maximum, that is to say at low automatic gain control voltages. Theconsiderations here are the same as those applying to the frequencyconverter stages.

In audio frequency amplifier stages high impedance operation is oftennecessary, and the possibility of gain control on audio frequency byvarying the function of one of the grids in the tube is desirable. 1

In a detector stage it is desirable to provide a triode or highimpedance low frequency amplifier stage in the same envelope in thediode. If low frequency amplifier stages are used the magnificationshould be 5 or 6 times in the case of a triode, and as much as 40 ormore times in the case of a high impedance tube. However, small separatediodes are very easily made and are very cheap, and have certainadvantages in the circuit, so that they may be used instead of employinga combined detector and amplifying tube.

In a power output stage the tube must give an adequate output to operatea loud speaker with a peak voltage on the control grid of not more thanabout 15 tovolts. Distortion must be as low as possible, which impliesin practice that in the output stage the undesirable effects ofsecondary electron emission from the anode to the next electrode shouldbe reduced as much as possible.

Finally, as regards automatic gain control, the

circuit should be capable of, controlling the output and maintaining thediode voltage at a value not greater than 10 volts. Higher values tendto produce whistling, and of course, overloading of the intermediatefrequency amplifier. Distortion and cross-modulation must be kept at aminimum. As mentioned above, it is sometimes useful to apply theautomatic gain control'to an audio frequency tube as well as to theradio frequency tubes; Cost and reliability are basic considerations sothat consequently receivers generally are furnished with extremelysimple automatic gain control circuits that must operate adequately fromthe lowest input voltage to the receiver which will operate the diodeeffectively up to an input of as much as two or three volts which may beset up by a strong local station. Quiescent automatic gain controlcircuits giving interchannel suppression are desirable.

It will be appreciated therefore that the factors to be taken intoconsideration in producing a satisfactory universal tube for the purposeindicated are numerous and complicated, but nevertheless the presentinvention has solved the problem satisfactorily.

The universal tube is very attractive from the points of view ofmanufacture, servicing and use. In the first two directions it ispossible very greatly to decrease costs. From the point of view of theusing, it becomes very easy to obtain replacements compared with theexisting system of using a number of different types of tube in eachreceiver. t is not difficult to design a universal'tu'be, for example,by employing a large number of different elements in one bulb orenvelope. Such a tube, however, would be so complicated to manufacture,and probably so unreliable when manufactured as to be quiteimpracticable.

It is required not only to produce a universal tube, but to give with ita performance at least as high, or even higher than that obtainable withthe non-universal tubes commonly in use. The performance, as alreadyindicated, must include all the many various and complicated functionsof the modern superheterodyne receiver, while at the same time,the tubemust be cheap and simple enough to be employed in cheap and simplereceivers. The requirements that it must give automatic volume controlfrequency changing, diode detection, with straight line characteristicsand so forth are extremely difficult to fulfil in one simple tube, andit has been found that they are only fulfilled by quite a definite typeof tube construction which will be described hereinafter.

It will be seen that whereas it would not be difficult, as mentionedabove, to produce a universal tube by means of an expensive constructionembodying all the elements of a large number of different tubes'mountedin one envelope of a universal tube, it is very difficult to obtain asimple construction capable of performing all these very diversefunctions. It has however been found possible, in accordance with thepresent invention, to produce a tube which consists merely of anextremely simple construction of concentric grid electrodes around acathode and an anode enclosing said grids; these electrodes are soformed that when different potentials are applied to them, and when theyare connected in different ways, the electrical characteristics of thetube are changed. In this way the use of a number of different elementsof different types of tube in the one envelope is avoided.

Not only are a number of different special characteristics availablewith a simple tube, but these characteristics are actually those whichgive the best possible efficiency in each of the various stages ofmodern radio receivers, even of the most complicated type.

It may be mentioned that the requirements of each stage of a radioreceiver may be represented as optimum values. For example, there is anoptimum value of mutual conductance for each I stage, and it isdesirable for the anode to confore, lies in the extreme simplicity ofthe tube and of the circuits in which it is to be used, and theremarkable fact that such a simple arrangement gives all the desired.optimum characteristics for each stage of modern complicated radioreceivers. These desired optimum characteristics may be varied to suitgiven circumstances by varying the configuration, including the meshspacing of the electrodes of the tube.

In view of the above considerations, according to the present invention,an electron discharge tube, having at least four concentric gridelectrodes around the cathode and an enclosing anode, has the secondgrid counting from the oathode, constructed with a more open mesh thanthe first grid from the cathode, while the third grid from the cathodeis made with a closer mesh than the first grid, and the fourth or anyother grid further from the cathode than the first three grids is madewith a mesh more open than any of those first three grids. The firstgrid counting from the cathode is constructed to have an appreciablecontrolling action on the anode current, its surface in the path of thedischarge being spaced from the cathode by a distance of the order of0.3 millimetre, and it is made with a mesh closer than of the order of10 turns per centimetre, and consists of wire having a diameter of about0.1 millimetre; the third grid from the cathode has a mesh closer thanof the order of 10 turns percentimetre, and is made of wire also havinga diameter of about 0.1 millimetre, so that it also has an appreciablycontrolling action on the anode current; the second grid, also made ofwire of a diameter of about 0.1 millimetre, has a mesh not closer thanabout of the order of the 10 turns per centimetre, while the fourth gridhas a mesh not closer than of the order of about 8 turns per centimetre.The distances between the surfaces of the grids in the path of thedischarge are approximately equal and of the order of 1 to 2millimetres.

In the case of a five-grid tube the fifth grid, which is placedimmediately next to the third grid counting from the cathode, is made ofsubstantially the same size wire and with the same mesh as the thirdgrid.

The grid nearest to the cathode, which, when the tube is used in asingle stage frequency changer circuit, is connected as the oscillatorycontrol grid, has its lead taken out of the envelope of the tube at theopposite end to that at which the leads of the remaining electrodes aretaken out. The grid next to the cathode and the third grid from thecathode have substantially the configuration mentioned above, and themutual conductance for the former grid is not less than of the order of1 milliampere per volt, and for the latter grid not less than 0.25milliampere per volt. The arrangement may be such that in the absence ofa metal screen placed externally to, and in close proximity to theenvelope of the tube, the capacity between the grid next to the cathodeand the anode is not less than of the order of 0.007 to 0.001 mmfd.

The grid electrodes have an elongated shape in transverse section, andfor convenience of manufacture their profiles may be given the shape ofcircular arcs. A dished electrostatic screen, extending into closeproximity with the inner wall of the envelope, is attached to the outerend of the grid assembly so as to shield the grid nearest to the cathodefrom the outer surface of the anode.

In producing a tube particularly suitable for the purpose set out above,it has been found of .advantage to set the anode of the tube at a.distance from the nearest grid electrode which is substantially thecritical distance in the sense defined in my patents, Nos. 2,045,525,2,045,526 and 2,045,527. In said patents, it is explained that if theanode of a tube were placed at various distances from the electrodenearest to it, the positions and spacings of the other electrodes andthe operating constants of the tube being otherwise unchanged, a' curvecould be plotted showing the relationship between the varying distancesof the electrode and what is termed the breakdown voltage, that is theanode voltage at which the anode current reaches its saturation value.Such a curve shows that if .the anode distance is reduced from rather alarge value, the breakdown voltage decreases to a minimum but that itincreases again as the anode is moved nearer to the adjacent electrode.

This result is due to the effect of secondary electron emission from theanode. The distance apart of the anode and the next electrode yieldingminimum breakdown voltage is termed the critical distance in saidpatents, and this is the sense in which the expression critical distanceis used in this present specification. It is also shown in my said priorpatents that the characteristics of the tube are sometimes improved byarranging that the positive potential on the grid electrode nearest tothe anode is lower than that on grids further from the anode.

Minor modifications may be made in the tube to suit variouscharacteristics. Thus, the mesh of the various grid electrodes, that isto say,'the number of turns per centimetre in the coils forming thegrids, may be varied, and the spacing may be varied to suit differentconditions. Thus, the grid nearest to the cathode may be of the sharpcut-off type and any of the other grids, particularly for use in anautomatic gain control electrode, may, if desired, be constructed tohave a variable-mu characteristic.

The invention will now be more fully described with reference to theannexed drawings, which Show an embodiment of a tube, in accordance withthe present invention, with certain explanatory diagrams and certaincircuit diagrams in which the tube may be employed, and in which:-

Figure l is an elevation with the external screening shield shown insection and parts of the other electrodes cut away to show the detailsof construction;

Figure 2 being shown approximately three times their actual size;

Figure 3 is a circuit diagram showing the tube connected as a singletube frequency changer;

Figures 4, 5, and 6 show different characteristics of the tube whenoperating under the conditions shown in Figure 3; i

Figure '7 shows the tube connected as an intermediate frequencyamplifier;

Figures 8, 9, and 10 illustrate different characteristics of the tubeutilized in the circuit diagram of Figure 7;

Figures ll, 12, and 13 show different characteristics of the tubeutilized when the tube is connected as a radio frequency amplifier;

Figure 14 shows the tube connected as an efiective tetrode acting as adetector amplifier;

Figure 15 is a circuit diagram of the tube connected in a power outputstage;

Figures 16 and 17 show characteristics of the tube utilized in thecircuit shown in Figure 15;

Figure 18 is a circuit diagram of the tube connected as an efiectivetriode acting as a detector amplifier; a

Figure 19 is a circuit diagram of the tube connected as a plain triode;while Figure 20 is a complete circuit diagram of a supersonicheterodyne' radio receiver, comprising four of the universal tubesconnected respectively as in Figures 3, 7, l4 and15.

In Figures 1 and 2fu1l details are shown of a tube with a cylindricalanode a, an indirectly heated cathode c of the usual British type with a4-watt heater, and five grids between these two electrodes, viz., g g 9g and g In the actual sample, the cathode c is rectangular incross-section, the sides being 1.5 milimetres and 1 millimetre in'length. The diameter of the anode a. may be taken as 27 millimetres andthe rest of the dimensions in' Figures 1 and 2 are to scale. It will benoted that in plan view the first grid g is of flattened shape, whilethe rest of the grids appear as two circular arcs passing around thesupports 3, all of which are nickel rods of a diameter of 0.75millimetre. The spacing of the electrodes may be varied to suitdifferent conditions, but in the sample described the respective radiiof curvature of the arcs of the grids g g g 9 are 10, 10.6, 11.5 and 14millimetres. The distances from centre to centre of the supporting rodsfor the grids g 9 g and g are respectively 10, l4, l8 and 22millimetres, while in the oase of grid 9 this distance is 6 millimetres.The minor axes of grids g g 9 g are respectively 3.7, 7.4, and 12millimetres and the parallel sides of the grid g are 2 millimetresapart. The mesh of the different grids may also be varied to suitdifferent conditions. In the sample taken, they vary from about 5.5turns per centimetre in the gridg to turns per centimetre in the grid gthe spacing in the grid 9 being 7 .1 turns per centimetre, that of thegrid g 12 and that of the grid g 14 turns per centimetre. .All the gridsare wound of molybdenum wire, the diameter of the wire of 9 being 0.08millimetre, that of the grids g g g*, 0.1 millimetre and that of thegrid 9 0.15 millimetre. The first grid 9 has a lead Z going to the upperterminal t. The lead for the grid g next to the anode is taken out at aside terminal 15 whereas the leads for the other three grids, the anode,cathode and heater are taken out to the seven pins p. The side terminalmay, of course, be omitted and a base used with one additional pin. Thegrids g g g g are wound uniformly, but if it is desired to provide avariable-mu or remote cut-off characteristic, one

of the grids, for example the grid g may have some turns omitted alongits length.

The screening is very simple and is effective because of its exactposition and the wide spacings involved. When the tube is used as avoltage amplifier and the anode to control grid capacity must be aminimum, an external screen s is employed fairly closely conforming tothe upper part of the glass bulb b. The internal screen consists of anupper screen e of dished shape with a hollow central portion 1 supportedon a mica bridge plate 9 extending across the electrode assembly. Thedished screen e extends approximately into, the neighborhood of theinner wall of the bulb b. There is also a lower hollow screen hsupported from a second mica bridge plate It and surrounding the lowerends of the grid assembly. A getter support is shown at m. In such atube, with the external screen s in position, the anode to control gridcapacity is about 0.001 mmfdn The anode is cylindrical and is widelyspaced from the outermost grid 9 as seen in Figure 2. It is of blackenednickel to reduce the secondary emission from it'and this tends toflatten the minimum portion of the distance curve. The anode is spacedsubstantially at the critical distance from the outermost grid g A tubeconstructed in the way described and illustrated in Figures l and 2 hasthe desirable properties of a universal valve as already set out above.In particular, the capacity between the grids g and g is small comparedwith that between the grids 9 and g the ratio between these capacitiesbeing such that when the grids g and g are connected as oscillatorelectrodes as will be described with reference to Figure 3, theoscillator circuits are not coupled to the radio-frequency input circuitto an undesirable extent and "locking-in is avoided. This desirableratio is obtained because the grid 9 is connected to the terminal t atthe top of the bulb b whereas the leads from the grids g and g are takenout at the lower end of the bulb. When a universal tube is to servewithout alteration as a screened amplifier as well as a frequencyconverter, the first grid 9 must have its lead taken out at the oppositeend from the other electrodes or the capacity between the grid 9 and theanode a underscreened amplifier conditions will not be low enough.

In Figures 3 to 19 of the drawings, some possible forms of connection ofthe tube when used for different purposes are illustrated.

In Figure 3, the connections of the tube as a single tube frequencychanger are shown. The grids 9 g operate respectively as the controlgrid and anode grid of theoscillator part of the Valve, the tunedoscillator circuit I being connected to the grid 9 and the anode circuitfeedback coil 2 being connected to the grid 9 The grid 9 is the inputgrid for the signal frequency and is connected directly to the tunedinput circuit 3. The grid 9 is the automatic gain control grid separatefrom the input grid and is connected directly to an automatic gaincontrol bus bar 4. Alternatively, the functions ofthe. grids g and 9 maybe interchanged. Either of these grids may be wound nonuniformly so asto give a variable-mu cut-off characteristic and then both the signalfrequency and the automatic gain .control voltages may be applied to thesame grid.

' The anode a is coupled to the next stage, for example, theintermediate frequency amplifier in the ordinary way.

The grid 9 is connected through a break-down resistance 5 and is apositive screening grid. The oscillator potentials on the grids g and gare in opposite phase and the ratios of the capacities between the gridsg and g and the grid 9 (or g") are such that the oscillator circuits I,2 are not coupled to the radio frequency circuit to anundesirable'extent. This method of balancing out the feed back is foundto be better than screening and is not affected by frequency. In thiscase, with the tube constants as described with reference to Figure 1,the operating conditions are as follows:-The anode voltage is 250 andthe break-down resistance 5 has a value of about 60,000 ohms so that asteady potential of I about volts is applied to the grids g and 9 Thecathode bias resistance R is 200 ohms and the grid leak resistance R15,000 ohms. The condenser 01 is 0.001 mfd. The anode current in theabsence of an automatic gain control voltage is from 1.5 to 2.0milliamperes. The cathode is at about 3 volts positive and the currentflowing to the grid 9 is about 9 milliamperes. The internal alternatingcurrent resistance of the tube is 1 million ohms. The conversionconductance with zero automatic gain control voltage is about 0.7 to 0.8milliampere per volt.

Certain characteristics utilized when the tube is connected as in Figure3 are shown in Figures 4 to 6. Figure 4 shows the anode current, anodevoltage characteristic with the internal alternating current resistanceat one million ohms, as mentioned above. The grid 9 owing to the form ofconnection shown in Figure 3, acquires a potential of 5 volts. to theautomatic gain control bus bar 4 in Figure 3, is connected to thecathode while obtaining the characteristic curve shown in Figure 4, soas to be at zero potential, whereas the grids g and g are connectedtogether and maintained at 100 volts. The current to the grid 9 is 0.04milliampere. The current to the grid 9 is 7 milliamperes. 1

- Figure 5 shows the characteristic with the anode current plottedagainst the varying potential of the input grid 9 The curve is takenwith the tube oscillating with an anode potential of 250 volts, the grid9 as in the case of Figure 4 being at cathode potential, the grids g and9 connected together maintained at 100 volts, and the grid 9 atapproximately 5 volts. The slope of the curve in Figure 5 is of courseproportional to the conversion conductance of the tube.

In Figure 6 the characteristic is shown with the anode current plottedagainst the voltage of the grid 9 with respect to the cathode as itwould be varied by the automatic gain control, in order to show thecut-off of the tube under these conditions. For the purpose ofdetermining this characteristic, the tube is operated without selfbias.The potentials of the anode and of the grids 9 g and. g are as in Figure5, while the input grid 9 is at about 2.8 volts.

In Figure '7, the connections are shown for a controlled gain voltageamplifier suitable for use in the intermediate stage of a supersonicheterodyne receiver. The tube is in effect a tetrode as the first grid gacts as the input grid, the third grid 9 as the automatic gain controlgrid, while the other three grids 9 and g are connected directlytogether and through a resistance 6 to the high tension source so thatthey act as positive screening grids. The anode to control grid capacitywith the external screen in position is about 0.001 mmfd. It will benoticed that here The grid 9 shown connected the amplitude distortionwhich accompanies the method of gain control by means of a variable-mucharacteristic, If the tube has the dimensions described with referenceto Figure 1, the following gives the operating conditions:-The anodevoltage is 250. The breakdown resistance 6 has a value of 60,000 ohmsand the cathode bias resistance R3 is 150 ohms. The voltage of the grids9 g and g is between 60 and '70 volts. If the grid 9 is at the samepotential as the cathode, the mutual conductance of the tube is of theorder of 2.2 milliamperes per volt and the anode current about 7.5milliamperes. The internal alternating current resistance of the valveis about 1 million ohms. It is important to note that as increasinglynegative automatic gain control voltages are applied to the grid 9 thecurrent flowing to the grid Q will increaseand therefore the screenvoltage on the. grid will be reduced.

The total current taken by the valve will also fall owing to the reducedpositive field acting on the cathode space charge with the particularconfiguration of the grids provided. Also, the cathode bias due to thevoltage drop across the resistance R3 will be reduced proportionally. Inthis way, the operating cathode bias remains at the correct value withrespect to the screen voltage at all values of the negative automaticgain control voltage on the grid 9 This is an important property of thetube when employed in this circuit. If methods of producing thenecessary grid bias had been used, other than a cathode resistance, thenthe. cathode or grid bias would not have taken automatically a suitablevalue. for all values of the automatic gain control voltage.

Certain characteristic curves obtained from the tube under theconditions shown in Figure 7 are illustrated in Figures 8 to 10, andillustrate the above stated facts. In Figure 8, two characteristiccurves, 20 and 2! are shown to illustrate the relation between the anodecurrent and the anode voltage. In the curve 20 the voltage applied tothe grid g is that of the cathode, and in curve 2| the voltage of thegrid 9 is 2.2.

Figure 9 is the characteristic curve showing thev anode current when theanode voltage is 250, as mentioned above, and the voltage of the grid 0is varied. The slope of this curve gives the mutual conductance which,as mentioned above, is of the order of 2.2 milliamperes per volt.

In Figure 10 the curve illustrates the anode current with the voltage ofthe grid 9 varied and the voltage of the grid g maintained a cathodevoltage.

Further characteristic curves of the tube are shown in Figures 11 to 13,when the tube is connected as a voltage amplifier for use in a radiofrequency stage. Figure 11 comprises two anode current, anode voltagecharacteristics 22, 23, the former being taken with the grid g at 4.5volts and the latter with the grid 9 at 7 volts with respect to thecathode. Figure 12 shows the anode current plotted against the voltageof the grid g Figure 13 comprises three curves, 24, 25 and 26. The curve24 shows the conditions when the grid g is maintained at cathodepotential, the voltage of the grid 9 being varied and plottedhorizontally. The curve 25, on the other hand, shows the Voltage of thegrid 9 maintained at cathode potential and that of the grid 9 varied andplotted horizontally. The curve 26 shows the conditions when the grids gand g are kept at the same potential, which is varied as shown by thehorizontal scale.

In Figure 14, the tube is shown connected as a single valve detectoramplifier, the tube serving as a tetrode. The anode a of the tubeoperates as a diode on a virtual or floating spacecharge cathode formedbetween the grids and anode. The grids g g and g are connected togetheras positive screening grids, being connected to a potential divider R4,R5 across the high tension source, while the grid 9 serves as the anodeof the amplifier part of the valve. This form of connection gives amagnification of up to the order of 40 times. The automatic gain controlconnection is made at 8, for example,

to the line 4 in Figures 3 and '7. If, again, the tube has thedimensions as in Figure 1, the operating conditions are as fol1ows:-Theanode voltage is 250. The voltage divider resistances R4 and R5 arerespectively 250,000 and 50,000 ohms so as to produce a potential ofabout 40 volts on the grids g g g The resistance Re which serves as theresistance coupling the valve to the next stage has a value of 30,000ohms, while the cathode bias resistance R1 is 1,000 ohms. The grid leakresistance Rs may be 1 million ohms. The diode load resistance R9 is500,000 ohms and the automatic gain control filter resistance R10, 1million ohms. The eificiency of rectification is high, of the order of96 per cent.

In Figure 15 the tube is shown connected to act as a power output tube.The external screen is not used but the tube so connected has a lowanode to control grid capacity. The grid g is the input grid and thegrids g, g and g are positive screening grids connected to a potentialdivider R11, R12 connected across the high tension source 1. A loudspeaker I0 is shown transformer-coupled to the anode circuit of thetube. With the tube illustrated in Figure 1, the operating conditionsare as follows:-The steady potential of the anode a and of the grids gand g is 250 volts and that of the grid 9 is about 70 volts. The cathodebias resistance R13 is 250 ohms. The anode current is 32 milliamperesand the mutual conductance of the order of 3 milliamperes per volt. Thecathode bias is about 12 volts and the tube should be capable of givinga power output of the order of 2 to 3 watts with a load of about 6,000ohms.

In Figures 16 and 17 certain characteristic curves of the tube are shownwhen operating under the power output conditions illustrated in Figure15.

In Figure 16 the three curves 21, 28 and 29 are three anodecurrent-anode volt characteristics with the grid 9 maintainedrespectively at the cathode potential, -12 volts and 24 volts withrespect to the cathode. The curve 30 shows that the total currentflowing to the screens plotted vertically against the anode voltsplotted horizontally in all the curves in Figure 16 the grid g is keptat cathode potential.

Figure 17 shows the relation between the anode current and the voltageof the grid g In Figure 18, the tube is shown connected as a single tubedetector amplifier actually operating as a triode. g is the input grid,the anode a acts as a diode and the output is taken ofi from theremaining four grids g 9 g and g con nected together. With the tubeshown in Figure 1, the operating conditions are much the same as statedin connection with Figure 14, except that the potential divider R4, R isomitted andthe coupling resistance R14 may conveniently havea hig hva1ue, for example, of 50,000 to 100,000 ohmsj Figure 19 shows the tubeconnected as a plain triode employed for example as an output tube. Thegrid 9 is the input grid, the remaining four grids g 9 and g are allconnected direct to the anode a and with it formthe output electrode.

- Figure 20 shows the circuit connections of a complete supersonicheterodyne receiver. The tube 1: is connected as a single-tube frequencychanger precisely as shown in Figure 3. The tube 12 is an intermediatefrequency amplifier connected exactly as shown in Figure 7. The tube 12is a combined detector and amplifier connected as a tetrode' "inprecisely the manner shown inFigure 14,while the tube 12 is a poweroutput tube connected exactly as shown in Figure 15. The circuit isshown with the ordinary antenna tuning arrangements and the ordinarymains power unit for the high tension supply with a winding ll supplyingthe current to the heaters of the cathodes of the tubes. Also thefrequency changer tube 1.1 is shown with switching arrangements forchanging the range of wave-lengths. The circuit connections will beapparent after examination of Figures 3, 7, 14 and 15 sincethe samereference characters have been used for corresponding parts.

It will be easily appreciated that the same form of tube could not" beused in all the circuits if they had not the following characteristicfeatures. In the case of'the frequency changer tube 11 the first grid 9has its lead taken out at the top of the bulb b and is prevented fromproducing serious lock-in by means of the capacity ratio of the grids asdescribed instead of by shielding. .Since the first grid g is connectedto the terminal t at the top of the bulb b, that is to say, at theopposite end to that at which the anode a is connected, the sameconstruction of tube can therefore be used as the intermediate frequencyamplifier '0 By the provision of several grids it is possible to employthe same construction of tube as the combined diode and tetrodeamplifier v The potential of the grid g while sufiiciently high to serveas an anode break-down voltage, low compared with the voltage of thehigh tension source used in the receiver and compared with the voltageof the anode a and the other positive grids, is nevertheless low enoughto allow of a critical anode distance within the dimensions of a bulb ofconvenient size so that the advantages of the anode critical distance asregards power output and low distor tion level are retained.Furthermore, owing to the provision of a number of grids, there is agrid in the case of the frequency converter and intermediate frequencyvalves, which is nearer the anode than the control or oscillatory gridsand is available as an automatic gain control electrode.

I claim:

1. An electron discharge tube having a cathode, at least five concentricand successive co-extensive grids enclosing said cathode and an anodeenclosing said grids and being spaced at the critical anode distancefrom the nearest of said grids, the first grid counting from the cathodehaving an appreciable controlling action on the anode current, the gridsurface in the path of the discharge being spaced from the cathode by adistance of the order of about 0.3 mm. and having a mesh closer than theorder of 10 turns per cm. of 0.1 mm. diameter wire, the third and fourthgrids counting from the cathode havbeing of mesh not closer than aboutthe order of 10 turns per cm. of .1 mm. wire, the fifth grid having meshnot closer than the order of about 8 turns per cm. the distances betweenthe faces of the grids in the path of the discharge being approximatelyequal and of the order of 1 to I 2. An electron discharge tube accordingto claim 1 characterized by screens internal and external of the tubeenvelope, the internal screen having a part of its surface close to thewall of v the envelope and approximating in curvature to the curvatureof the envelope and the exterior screen enclosing and fitting closely tothat part of the envelope close to said part of the surface of saidinternal screen.

3. An electron discharge tube according to claim 1 in which certain ofthe grid electrodes are oval in contour and have their profiles definearcs.

4. An electron discharge tube according to claim 1 characterized byhaving the control electrode so spaced from the cathode and of such meshthat the mutual conductance is of the order of at least one milliampereper volt.

5. An electron discharge tube accordingto claim 1 characterized byhaving the grid electrode nearest the cathode so spaced therefrom and ofsuch mesh that with positive potentials of the order of 250 volts on thesucceeding electrodes, the anode current is not less than the order of30 milliamperes.

6. An electron discharge tube having a cathode, at least four concentricand successive co-extensive grids enclosing said cathode and an anodeenclosing said grids and being spaced at the critical anode distancefrom the nearest of said grids, the first grid counting from the cathodehaving an appreciable controlling action on the anode'current, the gridsurface in the path of the discharge being spaced from the cathode by adistance of the order of about .3 mm. and having a mesh closer than theorder of 10 turns per cm. of 0.1 mm. diameter wire, the third gridcounting from the cathode having its mesh closer than the order of 10turns per cm. of .1 mm. diameter wire thereby having appreciablecontrolling action on the anode current, the second grid counting fromthe cathode being of mesh not closer than about the order of 10 turnsper cm. of .1 mm. wire, the fourth grid having its mesh' not closer thanthe order of about 8 turns per cm. the distances between the faces ofthe grids in the path of the discharge being approximately equal and ofthe order of 1 to 2 mm.

.7. An electron discharge tube according to claim 6 characterized byscreens internal and external of the tube envelope, the internal screenhaving a part of its surface close to the wall of the envelope andapproximating in curvature to the curvature of the envelope and theexterior screen enclosing and fitting closely to that part of theenvelope close to said part of the surface of said internal screen.

8. An electron discharge tube according to claim 6 in which certain ofthe grid electrodes are oval in contour and have their profiles definearcs.

9. An electron discharge tube according to claim 6 characterized byhaving the control electrode so spaced from the cathode and of such meshthat the mutual conductance for said control grid is of the order of atleast one milliampere per volt.

10. An electron discharge tube according to claim 6 characterized byhaving the grid electrode nearest the cathode so spaced therefrom and ofsuch mesh that with positive potentials of the order of 250 volts on thesucceeding electrodes, the anode current is not less than the order of30 mi1liamperes.

11. An electron discharge tube having a cathode, at least fourconcentric and successive grids enclosing said cathode, and an anodeenclosing said grids, the second grid counted from the cathode having amesh more open than that of the first grid counted from the cathode, thethird grid counted from the: cathode having a mesh closer than that ofthe first grid and a grid further from the cathode than said first threegrids having a more open mesh than any of the said first three 12.An'electron discharge tube according to claim 11 wherein the fourth gridhas a mesh of the same order as the third grid.

13. An electron discharge device according to claim 1 wherein the tube,when employed with the second, fourth and fifth grids at potentialsbelow about 80 volts, has an anode current of the order of not more thanmilliamperes and an anode resistance to alternating current, with anoperating anode potential of the order of 250 volts, of the order of 1megohm, the third grid being grounded and the control grid having anegative potential ofthe order of 1 volt thereon, and a mutualconductance of the order of 1.5 milliamperes per volt and so that whenthe fourth grid is grounded, the anode current is of the order of notmore than three milliamperes, the mutual conductance of the said thirdgrid is of the order of one-quarter to 1 milliampere per volt, the anoderesistance to alternating current and the mutual conductance of thefirst grid being the same and further, so that with a potential of 2 50volts on the anode and the second and third grids the negative potentialof the order of 10 to 20 volts on the first gridwith the fourth gridgrounded and a potential of the order of '70 volts applied to the fifthgrid, the mutual conductance is of the order of 2 to 4 milliamperes pervolt and the anode current is of the order of 30 to 40 milliamperes, theknee of the anodevoltage anode-current characteristic occurring at ananode voltage not greater than about 100 volts.

14. An electron discharge tube having a cathode, at least fiveconcentric and successive coextensive grids enclosing said cathode andan anode enclosing said grids and being spaced at the critical anodedistance from the nearest of said grids, the second grid counted fromthe cathode having a mesh more open than that of the first grid countedfrom the cathode, the third grid counted from the cathode having a meshcloser than that of the first grid, the fourthv grid counted from thecathode having a mesh closer than that of said first grid and the fifthgrid having a more open mesh than any of said other grid electrodes,said grids being so related that the tube, when employed with thesecond, fourth and fifth grids at potentials below 100 volts, has ananode current of the order of not more than 10 milliamperes and an anoderesistance to alternating current, with an operating anode potential ofthe order of 250 volts, of the order of 1 megohm, the third grid beinggrounded and the control grid having a negative potential of the orderof 1 volt thereon, and a mutual conductance of the order of 1.5milliamperes per volt and so that when the fourth grid is grounded, theanode current is of the order of not more than three milliamperes, themutual conductance of the said third grid is of the order of one-quarterto 1 milliampere per volt, the anode resistance to alternating currentand the mutual conductance of the first grid being the same and further,so that with a potential of 250 volts on the anode and the second andthird grids, a negative potential of the order of 10 to 20 volts on thefirst grid with the fourth grid grounded and a potential of the order of'70 volts applied to the fifth grid, the mutual conductance is of theorder of 2 to- 4 milliamperes per volt and the anode current is of theorder of 30 to 40 milliamperes, the knee of the anode-voltageanode-current characteristic occurring at an anode voltage not greaterthan about 100 volts.

JOHN HENRY OWEN HARRIES.

